The surface metal oxide species on oxide supports play a crucial role in the catalytic processes of supported metal oxide catalysts, which have been widely used as catalysts in numerous industrial applications: MoO.sub.3 /g-Al.sub.2 O.sub.3 and WO.sub.3 /g-Al.sub.2 O.sub.3 catalysts for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN),.sup.1-2 V.sub.2 O.sub.5 TiO.sub.2 catalysts for o-xylene oxidation to phthalic anhydride.sup.3,4 and selective catalytic reduction (SCR) of NO.sub.x.sup.5. The industrial development of supported metal oxide catalysts over the past five decades has been summarized in a recent review paper.sup.6.
Fundamental information about the surface metal oxide molecular structures have been obtained by a battery of physical and chemical techniques, including Raman spectroscopy, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), UV diffuse reflectance spectroscopy (UV-vis), solid-state nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS), Mossbauer spectroscopy, surface acidity, adsorption, and probe reactions..sup.7-10. The reactivity of the surface metal oxide species in various supported metal oxide catalysts has been probed by different chemical reactions including methanol oxidation, alkane oxidation, SO.sub.2 oxidation, and the selective catalytic reduction of NO.sub.x.sup.6. Correlation of the catalytic reactivity with the corresponding molecular structural information about the surface metal oxide species has elucidated many fundamental issues about the catalytic properties of the surface metal oxide species during catalytic reactions: the roles of terminal double bonds, bridging bonds, adjacent or neighboring sites, secondary metal oxide additives, support ligands, and preparation methods. The fundamental information obtained from these molecular structure-reactivity relationships has great potential for the molecular design and engineering of supported metal oxide catalysts for various catalytic applications.
The formation of a two-dimensional metal oxide species on surfaces of oxide supports through thermal spreading of three-dimensional bulk metal oxides (schematically represented in FIG. 1) is well documented in the catalysis literature.sup.7,8. Thermal spreading is a spontaneous process from a thermodynamics perspective. However, its kinetics are constrained and require a high temperature for surface diffusion or migration of one metal oxide component over the surface of a secondary oxide support to occur at an appreciable rate. In the context of thermal spreading, Tammann temperature (T.sub.Tam &gt;&gt;0.5 T.sub.mp ; T.sub.mp =bulk melting point of the dispersed metal oxide) is often used to estimate the temperature for thermal treatments. The driving force for thermal spreading and formation of the surface metal oxide monolayer is a concentration gradient of the dispersed component or a decrease in the overall system surface free energy.
In contrast, little information is available on the spreading of metal oxides over oxide supports during catalytic reactions. Gasior et al..sup.11 previously reported the spreading of vanadia over the surface of TiO.sub.2 (anatase) grains in a V.sub.2 O.sub.5 and TiO.sub.2 (anatase) physical mixture during o-xylene oxidation at 360.degree. C., which was manifested by increase in both conversion and phthalic anhydride selectivity with reaction time. Cavalli et al. earlier observed that soluble bulk V.sub.2 O.sub.5 spreads over the free rutile surface and, on the contrary, the insoluble vanadium oxide partly segregates and forms the soluble bulk V.sub.2 O.sub.5 during ammoxidation of toluene to benzonitrile at 320-390.degree. C..sup.12 However, the reaction temperatures (.about.360 .degree.C.) were much higher than the Tammann temperature (210.degree.C.) of crystalline V.sub.2 O.sub.5, implying that thermal spreading might have dominated the spreading kinetics during the reaction.